It finds applications in particular in the fields of digital radio communications, for example for identifying a transmitter, a type of transmitter or a type of transmission, in order to optimize multiple-mode transceivers.
In the context of digital transmissions, there is a need to be able to insert a secondary information stream into a main information stream.
Unlike, for example, the methods of marking video signals or audio or image signals, which are known from the prior art in relation to the transformation of analogue signals or signals sampled in the time-frequency space, the proposed method works by the insertion of a secondary stream comprising binary information (bits) into a main stream comprising symbols of a digital modulation.
The document WO 02/096051 (INFINEON) discloses a method for indicating to a receiver, for which a main symbol stream of a digital modulation is intended, the type of modulation concerned. The relevant secondary information is inserted in the form of shifts in the phase of the symbols of the sequences of training symbols of the bursts of the stream to be transmitted, which are assumed to be known in advance by the receiver. A major drawback of this method is that the secondary information thus transmitted in the main stream has a very low bit rate. Another drawback is that the phase shifts of the training symbols interfere with the channel impedance matching by the receiver.
From the prior art, various methods are also known for increasing the rate of a stream of information transmitted with a standard modulation, by using the standard symbols of this modulation and certain additional symbols.
For example, U.S. Pat. No. 6,373,903 (ERICSSON) discloses a method for transmitting a first binary stream and a second binary stream. The phase transitions associated with the symbols of the first and second binary streams are combined to form a sequence of modified phase transitions, which sequence is submitted to a modulator for transmission. In an embodiment (see FIG. 2 of the document), the second stream defines modifications of the phase transitions derived from the first stream: for each phase transition of the first stream in succession, an additional phase deviation about the cumulative phase states of the first stream is added according to the cumulative phase states of the second stream. This is applied individually to each symbol of the first stream independently of the preceding or next symbol, in the first stream. The following drawbacks arise from this. The average phase error of the modulation of the first stream is permanently increased, because it is directly proportional to the value of the additional phase deviations provoked by the insertion of the second stream. The upper limit of the length of the phase trajectories of the resultant modulated signal is increased, because the greatest phase transition (standard phase transition plus the additional phase deviation) can be greater than the maximum phase transition associated with a symbol of the standard modulation. The result of this is that the spectral properties of the resultant stream are degraded compared to the first stream. The demodulation of the resultant stream is therefore less effective than the demodulation of the first stream without insertion of the second stream. Furthermore, the additional phase shifts in the resultant stream transmitted are similar when replaying one and the same sequence of the second stream.
U.S. Pat. No. 6,426,978 (ERICSSON) discloses a method based on a coded modulation technique, for transporting two binary streams. A drawback is that this method virtually imposes the encoding, and therefore the type of modulated signal transmitted. It is therefore not applicable generally, when the insertion method has to be derived from a standard digital modulation.
Finally, from the document WO 03/005652 (INTERSIL), a method is known for switching from a single-carrier modulation (for example, a binary phase shift keying (BPSK) or quaternary phase shift keying (QPSK) modulation to a multiple carrier modulation (orthogonal frequency division multiplexing, OFDM, for example) without requiring the multi-mode receiver to re-evaluate all the applicable transmission parameters immediately after the switch between the two modulation schemes. Nevertheless, this method imposes compatibility constraints between the single-carrier transmission and the multiple carrier transmission, in order to ensure an uninterrupted transition. These constraints concern not only the automatic gain control, the similarity of the frequency-domain spectrum of the carrier and the phase, but also the equalization of the propagation channel, assuming that the estimation of the channel impulse response by the single-carrier demodulation is reusable for the multiple carrier demodulation, as well as the sampling and filtering and time-domain synchronization. A drawback of this method originates from the fixed nature of these constraints, and in particular, the similarity constraints in the frequency and time domain between single-carrier and multiple carrier transmissions. In practice, this fixed nature limits the possibilities of pairing between the single-carrier and multiple carrier modulations, because of the diversity of the possible parameters of the existing multiple carrier modulations.